1. Field of the Invention
This invention relates to electronic systems for detecting unauthorized movement of articles and more particularly it concerns novel arrangements which isolate such systems from the effects of metal objects and otherwise detectable targets which are close to but not within a predetermined interrogation zone.
2. Description of the Prior Art
Electronic article surveillance systems have been proposed as early as 1934 in French Patent No. 763,681 to Picard. According to Picard there are provided an interrogation loop antenna and a detection loop antenna adjacent an interrogation zone. The interrogation antenna is energized to generate a continuously alternating magnetic field in the interrogation zone. Protected articles are provided with elongated thin strips of an easily saturable magnetic material such as Permalloy which disturbs the magnetic field in the interrogation zone in a distinctive manner by producing characteristic pulses. The detection loop antenna converts the magnetic fields in the interrogation zone to electrical signals and these signals are processed in a receiver system to produce an alarm in response to the occurrence of the characteristic pulses. A modern and commercial magnetic detection system which contains many improvements over the original Picard system is described in U.S. Pat. No. 4,623,877.
In order to generate an interrogation signal of sufficient magnitude for reliable target detection without expenditure of excessive power, the interrogation antenna of most electronic article surveillance systems is arranged as an inductive loop and is connected with a capacitor to form a resonant circuit. In this manner when the system operates at the resonant frequency of the antenna-capacitor combination, a very strong interrogation signal can be produced with minimum expenditure of power.
A problem has been encountered in connection with electronic article surveillance systems which use resonant interrogation antennas. Specifically, these antennas are often used near large metal objects such as checkout counters and the like. The metal objects change the effective inductance of the antenna loop and consequently shift the resonant frequency of the antenna circuit. To some degree it is possible to compensate for this shift by changing the capacitance of the capacitor. However adjustable capacitors are expensive and the range of adjustment is limited. Also, installation of the system requires trained technicians to take proper measurements and make the necessary adjustments; and these measurements and adjustments must be repeated whenever the antenna is moved or whenever there is any change in the amount or nature of the metal objects adjacent the antenna.
Another problem encountered in connection with electronic article surveillance systems is the confinement of the system sensitivity to the interrogation zone. In supermarkets and similar mass retailing operations, the interrogation zone is immediately adjacent a checkout counter and/or conveyor along which legitimately purchased articles must pass. In some instances, the targets on the legitimately purchased articles are deactivated or removed before the articles move along the conveyor adjacent the interrogation zone. However in many instances the targets are not removed or deactivated and it is necessary that these targets be isolated from the detection system so that they do not become detected as they move along the checkout counter behind the interrogation antenna.